Multi-color electrostatic printers often employ a five pass process during which plurality of toners are placed on a photoconductive drum and then transferred to a paper carrier. During the first four passes, yellow, magenta, cyan, and black toners are applied, and on the fifth pass, the combined toners are transferred from the drum to a paper carrier.
In laser-based electrostatic printers, an electrostatic drum is first charged to a negative potential, after which the laser scans across the drum and discharges its surface where toner is to be placed. A developing roller coated with toner is generally charged to a negative potential that is intermediate between the original negative potential on the drum's surface and a totally discharged drum surface. Thus, the toner is attracted to the discharged areas of the drum surface and is repelled by the areas that are still negatively charged. Once a first toner is transferred to the drum surface, the drum is again charged and a new cycle of laser beam discharge occurs so that the next toner can be applied. Once all four toners are on the drum, transfer to paper occurs, as aforesaid.
As is known to those skilled in the art, primary colors are yellow, magenta, and cyan and secondary colors can be produced by a combination of the primary colors. The color red is produced by overlaying a magenta toner onto a yellow toner; green by overlaying a cyan toner onto a yellow toner; and blue by overlaying a layer of cyan onto a magenta toner. In general, when producing a secondary color, the "underprinted" toner will be yellow when printing magenta, yellow or magenta when printing cyan, and yellow, magenta, or cyan when printing black. While "process" black can be employed by combining the yellow, magenta, and cyan colors, it is often found more convenient to use a separate black toner to improve print quality.
With secondary colors, an underprinted toner previously placed on the electrostatic drum will reduce the power of the laser beam that strikes the drum surface. This results in a reduced level of discharge on the drum surface and a residual small negative potential. The residual negative potential repels some of an overprinted toner and causes a less dense layer thereof to be deposited on the drum surface. Thus, reds and greens appear somewhat yellowish and a blue color will be somewhat magenta-ish.
The Konica Corporation, in a system that is nonpublic at the date of filing hereof, but which is prior art hereto, has suggested that the laser beam, in preparation for deposit of an underprinted toner, be modulated so that the duration of exposure of the drum surface is less than the duration of exposure during an overprint application. That modulation enables the amount of underprinted and overprinted toners to be matched. More specifically, by providing a shorter period of exposure only during deposit of the underprinted toner, a somewhat smaller and less dense underprinted toner pixel is laid down and a somewhat larger and more dense overprinted toner pixel is superimposed thereover. This technique works well for producing a desired secondary color within a solid color-fill area, however, a property exists at the edges of the solid fill area that creates additional problems.
Due to properties of the electrostatic printing process, an underprinted toner of a secondary color tends to be drawn away from an edge and towards a solid fill area, more than an overprinted toner. This phenomenon appears as a "halo" of overprinted toner around a solid-color fill area.
The Konica system referred to above further employs an edge enhancement technique. When edge pixels are recognized and, when they are produced in a secondary color (comprising two overlaid primary color toners), the underprinted toner exposure is increased in relation to the exposure level for non-edge underprinted toner to prevent the occurrence of a halo effect. When edge pixels are recognized in a primary color, the toner exposure is decreased in relation to the exposure level for non-edge primary toner to control the width of primary color lines. The application of such an edge recognition procedure and a resultant reduction in applied laser power, itself, creates additional problems. For instance, when very thin single pixel-wide lines or isolated pixels are identified as "edge" pixels, a reduction in laser power can result in insufficient discharge of the drum surface and a concomitant loss of the pixel image altogether. Furthermore, where a secondary color pixel abuts a primary color pixel, a reduction in the exposure may create an undesired color perturbation at the interface.
Accordingly, it is an object of this invention to provide an improved method and system for rendering of color boundaries in a multi-color electrostatic laser.
It is another object of this invention to provide an improved method for rendering secondary color pixels that are immediately adjacent primary color pixels.
It is still a further object of this invention to provide an improved method and system for rendering of isolated pixels and thin pixel lines in a printer system wherein edge compensation is employed.